Hair exposed to chemical treatments with a permanent waving solution, oxidation hair dye (for coloring), or hair bleaching agent (for bleaching) is damaged due to a partial loss of its component or structure. The damage of the hair caused by such chemical treatments tends to be severer by the heat of a drier, hair iron, or the like or physical stimulation resulting from daily hair care routine such as brushing. Chemical treatments such as permanent waving, hair coloring and hair bleaching are usually performed with intervals of several months. Due to physical stimulation during this period, there appears a large difference in the damage degree between a hair root portion which has grown after such a chemical treatment and a hair tip portion exposed thereto.
Application of an oxidation hair dye to the hair having two portions different in damage degree tends to cause uneven dyeing because the greatly damaged hair tip portion tends to be dyed deeply and the slightly damaged hair root portion is relatively poorly dyed. To overcome such a problem, there is a demand for the development of oxidation hair dyes capable of uniformly dyeing hair having portions different in damage degree.
Branched fatty acids are well known as raw materials for hair cosmetic compositions. Hair cosmetic compositions containing an iso-fatty acid (a branched fatty acid having a methyl group at the second carbon atom from the carbon atom at the end of the fatty acid chain) or an anteiso-fatty acid (a branched fatty acid having a methyl group at the third carbon atom from the carbon atom at the end of the fatty acid chain) are also known (Patent Documents 1 to 4). Fatty acids derived from lanolin contain such an iso-fatty acid or anteiso-fatty acid and hair cosmetic compositions containing such a lanolin fatty acid are also known (Patent Documents 5 and 6).
Patent Documents 1 to 3 give Examples of shampoos containing 18-methylicosanoic acid and cationic cellulose. Patent Document 4 gives Example of a hair bleaching agent containing a lanolin fatty acid and it describes that the agent can suppress hair damage and provide the hair with an excellent feel. Patent Document 6 also gives Examples of an oxidation hair dye and a hair bleaching agent, each containing a lanolin fatty acid.
It is however not known at all that addition of such a branched fatty acid and a cationic polymer in combination has an influence on dyeing properties of the resulting hair dye composition, in particular, an oxidative hair dye composition containing, as the branched fatty acid, an iso-fatty acid or anteiso-fatty acid having from 19 to 30 carbon atoms, or salt thereof together with a cationic polymer is excellent in uniform dyeing of hair having portions different in damage degree and also excellent in the durability of its effect (meaning that uniformity is kept even when the color of the hair fades out gradually by shampooing).    [Patent Document 1] EP-A-0483689    [Patent Document 2] JP-A-4-230614    [Patent Document 3] JP-A-6-87724    [Patent Document 4] JP-A-2005-206524    [Patent Document 5] JP-A-63-316712    [Patent Document 6] JP-A-2001-213739